I know there are some who pretend, that the idea of duration is applicable in a proper sense to objects, which are perfectly unchangeable; and this I take to be the common opinion of philosophers as well as of the vulgar. But to be convinc'd of its falsehood we need but reflect on the foregoing conclusion, that the idea of duration is always deriv'd from a succession of changeable objects, and can never be convey'd to the mind by any thing stedfast and unchangeable. For it inevitably follows from thence, that since the idea of duration cannot be deriv'd from such an object, it can never in any propriety or exactness be apply'd to it, nor can any thing unchangeable be ever said to have duration. (paragraph 11)
Since we cannot derive an idea of duration from unchangeable objects, and cannot "in a proper sense" apply it to those objects, the thought is that unchanging impressions are instantaneous. This, however, cannot be right, because Hume immediately goes on to say:
Ideas always represent the Objects or impressions, from which they are deriv'd, and can never without a fiction represent or be apply'd to any other. By what fiction we apply the idea of time, even to what is unchangeable, and suppose, as is common, that duration is a measure of rest as well as of motion, we shall consider afterwards.
In other words, we can (and do) apply the idea of time to unchanging objects by a "fiction". As he clarifies in Section 5, to which this refers:
But tho' it be impossible to shew the impression, from which the idea of time without a changeable existence is deriv'd; yet we can easily point out those appearances, which make us fancy we have that idea. For we may observe, that there is a continual succession of perceptions in our mind; so that the idea of time being for ever present with us; when we consider a stedfast object at five-a-clock, and regard the same at six; we are apt to apply to it that idea in the same manner as if every moment were distinguish'd by a different position, or an alteration of the object. The first and second appearances of the object, being compar'd with the succession of our perceptions, seem equally remov'd as if the object had really chang'd. To which we may add, what experience shews us, that the object was susceptible of such a number of changes betwixt these appearances; as also that the unchangeable or rather fictitious duration has the same effect upon every quality, by encreasing or diminishing it, as that succession, which is obvious to the senses. From these three relations we are apt to confound our ideas, and imagine we can form the idea of a time and duration, without any change or succession. (last paragraph)
This shows that, despite the idea's not properly applying, we do in fact distinguish out different 'moments' in our perception of unchanging objects. This does not sound like instantaneity. Note the importance of tracing the idea of time to its originating impressions. This clarifies what Hume is actually doing. The reason we cannot derive an idea of time or duration from an impression of an unchanging object is that there is nothing in such an impression identifiable as time: there's just an unchanging object. In a changing object, however, we recognize the succession involved in the change, and this is, according to Hume, whence we derive the idea of time. We can apply this idea to unchanging objects only by treating the unchanging object as if it had changed, i.e., the object in which there is no discernible succession as if it had discernible succession. But there is nothing in the impression that prevents us from attributing succession to it, by fiction. This would not be possible if these impressions were genuinely instantaneous.
But isn't there reason to believe that Hume thinks that all our perceptions ultimately break down into instantaneous impressions, in his discussion of infinite divisibility? Not at all. Hume's discussion of the divisibility of time requires us to conclude that there are (as it were) temporal points: but these temporal points are simply the smallest noticeable succession:
The same reasoning will prove, that the indivisible moments of time must be fill'd with some real object or existence, whose succession forms the duration, and makes it be conceivable by the mind. (Section III, last paragraph)
Hume does allow us to talk about instants - again, by fiction - but these are a matter of "loose" ideas, rather than impressions. On Hume's account we do not have, and cannot have, instantaneous impressions.